LOOP-LIKE LIGHT EXTINCTION IN THE PRECIPITATION FILM OF A PRESHEARED LYOTROPIC POLYMERIC LIQUID-CRYSTAL

This paper reports a novel light extinction pattern of polymers in a p olarizing microscope, termed as looplike extinction by the authors. Su ch a phenomenon is found together with the Maltese cross, in the preci pitation film of presheared poly(1,4-phenylene terephthalamide) (PPTA) solution, a popular lyotrophic polymeric liquid crystal. A plausible supermolecular structure is assumed, which comprises one +1 defect and a pair of -1/2 defects. The key structure is that besides the core of the nematic droplet, the matrix is also anisotropic due to preshearin g. Calculated birefringence patterns find good agreement with the expe rimental micrographs. Copyright (C) 1996 Elsevier Science Ltd.